A Signed Instruction Is Not Settlement: Finality for Agentic and API Payments

Introduction The payment path, whether a human treasury screen, a batch file, or an agent tool call, is now: payment instruction {amount, payee, rail} -> signed or tokenized submit -> PSP / core / card / CBDC engine -> funds move ]]> This document controls the last arrow. The computer may compute an instruction. The rail may authenticate the sender. Neither fact is authority for this amount to leave this account toward this beneficiary on this rail in this epoch. The profile uses the two-boundary chain in and the tool-dispatch objects in . It specifies only payment predicates those drafts leave as an enum: amount and currency binding, beneficiary and rail binding, mandate and purpose binding, idempotency as consume-once authority, and coexistence with ISO 20022, ACH, card capture, and instant-payment APIs.

How Payment Abuse Changed

Past: Steal the Instrument Historical payment crime stole the thing that was already money: a card number, a cheque, a batch file, a SWIFT token on a compromised treasury PC. Defense was instrument-centric. Reissue the card. Rotate the MAC. Watch for a file that did not match yesterday's template. The valuable object was a credential or a pre-built instruction the business had already approved as a class.

Present: Steal the Session that Can Pay Present abuse often leaves the instrument in place. The attacker takes an ERP session, a PSP dashboard cookie, an agent seat with payout.create enabled, or a poisoned invoice PDF that the AP bot is allowed to read. Strong customer authentication and OAuth are up. The worker is allowed to pay. The instruction it emits is not the one the mandate described. Business email compromise already showed the pattern without models: a lawful payer, a new beneficiary string, a valid signature on the channel. Agentic checkout and AP agents industrialize that pattern. Every tool-enabled seat is a treasury clerk that never sleeps and that will treat retrieved text as an instruction.

Future: Compute the Payment, Then Settle It The coming incident does not need the attacker to format ISO 20022. The model formats it. A reconstructed "we owe this vendor" join from mail plus ERP plus a prompt-injected statement becomes a Candidate Act that looks like ordinary automation. If the settlement engine accepts any authenticated submit from that worker, reconstruction becomes cash. This profile exists so that even a correctly signed, schema-valid, token-bearing instruction remains non-effective until the sink checks the act digest against current authority. Settlement is not a synonym for "the API returned 200."

Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here. Failure to establish current payment-finality authority MUST NOT be converted into permission to post, capture, release, or otherwise settle funds.

Problem Space A payment instruction is a Candidate Act with a consequence class that is almost never informational. Once the rail posts, reversal is a business process, not a protocol rollback. The failures this profile treats as first-class are:

Amount substitution after approval of 50.00 as 5,000.00 on the same beneficiary and rail.
Beneficiary substitution after approval of a named payee.
Rail substitution (internal book-transfer authority reused on an external instant rail).
Replay of a consumed instruction under a new idempotency key or on a second PSP.
Mandate or purpose drift (invoice-pay authority used for payroll or cross-border).
Agent or worker substitution (approved bot A, executing bot B).
Destination-account reuse after revocation or beneficiary-risk list update.

In each case some upstream control can be green: the OAuth token is live, the ISO message verifies, the mandate exists, the agent is in the allowlist. The missing question is the sink question: may this exact instruction settle now.

Existing Solutions and What They Do Not Bind

ISO 20022, SWIFT, ACH, and Instant Rails Messaging standards authenticate and structure instructions. They bind fields inside a message. They do not, by themselves, consume single-use authority at the posting engine, and they do not stop a second valid message that differs in one load-bearing field after a human or model "approved payments."

OAuth, API Keys, and Open-Banking Consents Tokens establish that a client may call a payment API. Consent often names an account and a period. Possession of the token is bearer-like with respect to every instruction that token can form. Act digest, beneficiary bind, and consume-once are application additions, not properties of the token.

3-D Secure, SCA, and Step-Up Payer authentication answers "is a human present for this merchant session." It does not hash the agent-generated split of amount and payee after the session, and it does not sit in a payout API that never sees a cardholder.

Ledgers, Blockchains, and Smart Contracts A chain can make a transfer final among participants. It does not decide whether the off-chain agent was authorized to construct that transfer. Posting a stolen-but-valid signed payload is still settlement. External anchoring MUST NOT retroactively authorize an act that was invalid at effectuation time.

Mandates, Standing Orders, and Vendor Master Data A mandate is a class approval. Vendor master data is a directory. Neither is an act-specific, amount-bound, nonce-bound capability at the sink. An agent that is allowed to "pay approved vendors" can still pick the wrong vendor row or the wrong invoice total.

Human Dual Control Maker-checker is the correct industrial instinct. It is slow, and agents exist to skip the queue. This profile does not delete dual control. It makes the checker a predicate that can be a human share, a policy share, or a threshold of both, and it still requires the sink consume the resulting authority.

What This Profile Adds

the instruction remains a Payment Candidate Act after the model, ERP, or file emitter produces it;
amount, currency, beneficiary, rail, and purpose are in the digest;
authority is non-bearer, sink-bound, epoch-bound, and consumed;
idempotency is consume-once of that digest, not a client-supplied string the attacker can refresh;
legacy rails remain the transport; the sink check is the gate;
hot-path envelopes exist so instant payments are not forced through a remote ledger round trip.

Industrial Applicability

PSP and Platform Checkout A commerce agent or checkout API already holds a capture token. The sink is the capture or refund call. Authority issued for capture 40.00 at merchant M MUST fail for 4,000.00 or merchant M2.

Bank Core, ACH, SEPA, and Instant Credit The sink is the posting service that would debit the payer and emit the clearing message. ISO 20022 remains the payload. The Finality Sink verifies the Payment Candidate Act before the message is considered released to clearing.

Corporate AP and Treasury Agents ERP and TMS tools already batch pay. An agent that reads invoices is the present-theft seat. The RAO-like purpose bind is invoice-pay. Payroll and tax rails are different consequence scopes.

Claims, Insurance, and Benefit Payout A claims agent proposing a payee from a repair invoice is a beneficiary-substitution surface. Settlement authority MUST bind payee account digest, not only claim-id.

Payroll and Marketplace Split Many small legs share one worker token. Each leg is its own Candidate Act. A digest for worker A / 1200.00 MUST not authorize worker B / 1200.00.

CBDC, Stored Value, and On-Us Books Whether the engine is a CBDC participant, an e-money ledger, or an internal book, the sink is the component that would change balances. This document does not specify a CBDC protocol. It specifies that balance change is effectuation.

Agentic Wallets and MCP Payment Tools payout.create on an MCP server is a tool call under and a payment act under this profile. Both checks are required when the tool would move value. Discovery of the server is not mandate authority.

How a Bank Would Pilot in One Quarter Pick one outbound API used by a new AP agent or by a single PSP payout route. Deploy the consume table next to the existing idempotency store. Issue authority only for that route's currency and ceiling. Leave the green-screen path on maker-checker. Measure posts without authority_id, digest mismatches, replay attempts, and added latency at p50 and p99. Expand rail by rail. This is a pilot, not a core replacement.

How a PSP Would Expose It to Platforms Offer an optional header carrying the authority object and the instruction digest the platform computed. If the header is absent, today's token behavior remains for legacy platforms. If the header is present, the PSP becomes a cooperating sink and rejects mismatch. Platforms that run agents SHOULD move to header-present. That is how the profile meets the industry without a flag day.

Terminology

Payment Candidate Act: A generated or selected instruction that would debit, credit, capture, refund, release, or otherwise change a balance, but has not been permitted to do so.
Settlement Sink: The component that would actually post the instruction to a ledger, clearing file, card acquirer, CBDC engine, or stored-value book. If that component can be skipped, it is not the sink.
Instruction digest: A hash over the canonical amount, currency, beneficiary, rail, account references, purpose, and other load-bearing fields. Changing any of them MUST change the digest.
Mandate: A class-level approval (vendor, merchant, account, ceiling). A mandate MAY be an input to PED validation. It MUST NOT by itself be settlement authority for a particular act.

Architecture

Settlement-time finality post once, consume authority | `-- FAIL -> no balance change ]]>

End-to-End Workflow

Instruction constructed by agent, API, file, or human UI. Status NON_EFFECTIVE.
Canonicalize load-bearing fields. Compute instruction digest.
Build PaymentCandidateAct including payer, accounts, amount, currency, beneficiary, rail, purpose, freshness, sink, epochs.
PED validates predicates, including mandate envelope and risk lists where required.
On fail: deny, optionally lock nonce, do not post.
On pass: commit Protected Validation Evidence.
Issue scoped Payment Finality Authority bound to the digest, sink, epochs, and single-use flag.
Transport the instruction on the legacy rail (ISO, ACH, card, HTTP) together with the authority reference or an attached object.
Settlement Sink verifies live fields against authority immediately before post.
Atomic consume + post, or deny with no post.
Record sink-side settlement evidence. Do not treat rail ACK as a substitute for consume.

Stage notes that implementations get wrong: Canonicalization MUST include amount scale. 150, 150.0, and 150.00 are the same act only if the canonical form says so. Implementations SHOULD normalize to a fixed scale per currency (two for EUR, zero for JPY unless policy says otherwise) before hashing. expires_at on the Candidate Act is not the rail cut-off time. It is the authority TTL. A SEPA instant that settles in two seconds still uses a short TTL so a queued instruction cannot sit overnight and post after a risk-list change. Evidence commitment is constitutive. A PED that returns ALLOW=true without a stored evidence_id has issued a boolean, not this profile. The sink MUST be able to resolve the evidence or MUST treat the authority as invalid. Consume and post SHOULD be one database transaction or an equivalent compare-and-swap. Consume-then- crash-then-repost without a durable consume bit is a replay window. Post-then-crash-then-fail-to- consume is a double-post window on retry. The safe pattern is: write consumed+pending, post, write posted; retries read the row.

Settlement Pseudocode hot_envelope_ceiling(act) and not cold_path_approved(act): return ESCALATE_OR_DENY(AMOUNT_ENVELOPE) evidence = COMMIT_PROTECTED_EVIDENCE(act) authority = ISSUE_SCOPED_AUTHORITY(act, evidence) return ALLOW(authority) function SETTLEMENT_SINK_POST(act, authority, live): live_digest = HASH(canonicalize(live)) if authority missing or integrity_fail(authority): return DENY(NO_OR_INVALID_AUTHORITY) if live_digest != act.instruction_digest: return DENY(INSTRUCTION_SUBSTITUTION) if authority.sink_id != THIS_SINK: return DENY(SINK_MISMATCH) if live.amount != authority.amount or live.currency != authority.currency or canonical_payee(live) != authority.beneficiary: return DENY(FIELD_MISMATCH) if expired(authority) or consumed(authority): return DENY(STALE_OR_USED) ATOMICALLY: consume(authority) advance_replay_state(act.freshness.nonce) result = post_to_rail(live) record_sink_evidence(act, authority, result) return result ]]> post_to_rail() is unreachable when any check fails. A 200 from an inner HTTP client after a failed check is non-conforming.

JSON Interoperability Profile Objects are UTF-8 JSON. additionalProperties MUST be treated as reject for load-bearing objects. Amounts SHOULD be decimal strings to avoid binary float drift. Canonicalization for the digest SHOULD be JCS or an implementation-defined stable form declared in the digest object.

PaymentCandidateAct Schema

Evidence and Authority

Sink Verify Allow and Deny

Complete Allow Transaction

Load-Bearing Bindings Authority for amount 150.00 / EUR / vendor-441 / SEPA_INSTANT / digest D1 MUST NOT authorize 1,500.00, USD, vendor-442, ACH, or digest D2. DISPLAY-equivalent mistakes in this profile are amount substitution, beneficiary substitution, and rail substitution. Book-transfer authority MUST NOT automatically authorize an external instant rail. Refund authority MUST NOT automatically authorize a new capture. Authority created under revocation epoch N MUST fail at N+1, including after a beneficiary-risk-list update.

Practical Feasibility: Latency and Legacy A profile that cannot sit on a 200 ms instant-payment path, or that demands a greenfield ledger, will not be deployed. This section states how the checks fit existing engines.

Where the Microseconds Go The load-bearing work at the sink is: parse live fields, hash the canonical instruction, verify a signature or MAC on the authority, compare digest and sink id, test consume-once, post. That is the same order of cost as an existing idempotency-key table plus an HSM MAC verify. Representative targets, not protocol constants:

hot path, local policy and local consume table: about 1-10 ms added to an already-authorized envelope;
card or PSP capture on the same host as the enforcer: similar;
cold path with fresh beneficiary risk, new country, or amount above envelope: tens to hundreds of ms plus any human share;
external transparency log or chain anchor: asynchronous after post, never on the enable path.

The architectural split is the same as in : hot path still verifies at the sink; cold path must not be skipped by timeout.

Hot-Path Envelope A cold evaluation MAY establish a bounded envelope: payer account A, rail SEPA_INSTANT, currency EUR, max amount 500.00, beneficiary set S, purpose invoice-pay, epochs (P,R), sink core-post-eu-1, valid-until T. Later instructions inside that envelope MAY use local validation. Any change of beneficiary, rail, currency, ceiling, or epoch MUST escalate. "Hot" means cached predicates. It does not mean skip consume or skip digest compare.

Coexistence with ISO 20022 and Clearing Files The Finality Sink SHOULD sit immediately before the component that marks an instruction released to clearing. The ISO message can carry an end-to-end id that equals or hashes to candidate_act_id. The authority object MAY travel in an accompanying API header, a private supplementary data block, or a side channel keyed by that id. Clearing houses that cannot parse the object still receive only messages the sink has already admitted. Batch ACH or payroll files are N Candidate Acts, not one. A file-level signature is channel authentication. Each row still needs a digest and consume, or a tightly bound envelope that lists row digests.

Coexistence with Card Acquirers Capture and refund APIs already have idempotency keys. This profile treats that key as insufficient if the body can change under the same key, and as insufficient if a second acquirer accepts the same logical act. The sink is the acquirer-facing submitter in the merchant or platform, or the acquirer itself where the acquirer agrees to verify the authority object.

Coexistence with Core Banking Cores already implement posting, holds, and double-entry. The feasible placement is a pre-post adapter: no new general ledger, no new rail. The consume table can be a compare-and-swap row next to the existing idempotency store. If the core cannot host the adapter, the adapter MUST be in-line so that bypassing it makes posting impossible.

Coexistence with Agent Runtimes For MCP and function-calling agents the payment tool handler MUST call SETTLEMENT_SINK_POST before any PSP SDK. A local Python or TypeScript enforcer that only checks allowlists is not this profile unless it consumes digest-bound authority. Combining this document with means: build AgentCandidateAct for the tool call and PaymentCandidateAct for the value move, or one object that satisfies both required sets.

Failure Modes that Legacy Staff Will Recognize Timeout toward the PED is hold-or-deny, the same instinct as "do not auto-approve an SCA fail." Duplicate file submission is consume-once, the same instinct as ACH duplicate detection, but keyed by digest not by filename. Beneficiary change after checker approval is field mismatch, the same instinct as maker-checker, automated. A pilot that only wraps the payout API used by the new agent, and leaves the green-screen treasury path on dual control, is a conforming incremental deployment. Alternate-path closure says the green-screen path MUST eventually grow the same consume semantics if it can emit the same rail message. It does not require a big-bang cutover on day one.

Informative Mapping onto ISO 20022 Credit Transfer The following mapping is informative and does not modify ISO 20022. It exists so a payments engineer can see that this profile is an admission gate, not a competing message.

candidate_act_id — may populate or hash into EndToEndId / InstructionId.
amount + currency — InstdAmt / EqvtAmt and Ccy.
beneficiary account_ref or account_digest — CdtrAcct.Id (digest preferred in authority objects that leave the host).
purpose.invoice_ref — RmtInf or Purp.
rail_id SEPA / SWIFT / FedNow — clearing channel already selected by the core.
authority_id — private supplementary data or an API header on the submit that precedes clearing release.

If InstdAmt changes after authority issuance, the live digest MUST fail. If only ReltdRmtInf narrative changes and policy treats remittance as non-load-bearing, implementations MAY exclude it from the digest. That exclusion MUST be explicit. Silent exclusion of creditor account is non-conforming.

Informative Mapping onto Card Capture amount and currency map to capture amount. beneficiary_id maps to merchant or sub-merchant. rail CARD maps to the acquirer route. purpose may carry order_id. A refund is a different act_type. Using a capture authority to refund, or a 40.00 capture authority to capture 400.00, MUST deny. 3DS or SCA results MAY appear as PED inputs. They MUST NOT appear as a reason to skip sink consume.

What This Profile Does Not Require It does not require replacing SWIFT, cards, or ISO 20022. It does not require a blockchain on the posting path. It does not require the model to detect fraud. It does not require every micropayment to do remote attestation. It requires that the component which can change a balance refuse to do so without current, consumed, digest-bound authority.

Cross-Border, FX, and Split Legs An FX payout is two load-bearing facts: the debit in currency C1 and the credit in currency C2 toward a beneficiary in administration J. Implementations MAY model this as one Candidate Act whose digest includes both amounts, both currencies, the FX rate or rate source epoch, and J, or as two acts that MUST NOT settle independently if the business meaning is a single payout. Silent settlement of only the cheap leg is a substitution. Correspondent or cover payments MUST NOT inherit authority issued for the customer credit. A cover bank is another sink. Authority sink-bound to core-post-eu-1 MUST fail at correspondent-post-us-9 unless a new act is authorized.

Dual Control and Threshold Authority Where policy requires maker-checker, the PED MUST treat missing checker shares as validation failure, not as a warning. Shares MAY be human approvals, independent policy engines, or a k-of-n MAC. The issued Payment Finality Authority SHOULD list the share identifiers. The sink MUST reject an authority whose share set no longer meets k. Agents do not replace the checker. An agent MAY be the maker. If an agent is also allowed to be a checker, that fact MUST be explicit in policy and SHOULD be excluded for FINANCIAL class above the hot envelope. Dual control that lives only in a UI, while an API posts without shares, is an alternate path and is non-conforming for that API.

Payment Threat Catalog

T1 Stolen PSP or core API token
T2 Stolen enterprise agent seat with payout tools
T3 Injected invoice or vendor-change mail
T4 Amount substitution after UI approval
T5 Beneficiary substitution / BEC
T6 Rail substitution (on-us to instant)
T7 Replay on a second acquirer or PSP
T8 Idempotency-key reuse with new body
T9 Mandate used off-purpose
T10 Payroll file row injection
T11 FX-leg split to move value cheaply
T12 Risk-list epoch rollback
T13 Checker UI bypass via API
T14 Agent A authority used by agent B
T15 Refund used as origination

The protocol does not require the model to label these attacks. It requires post_to_rail() to be unreachable when the corresponding bind fails.

Additional Worked Denials

Regulatory and Operational Notes This document does not claim to satisfy PSD2, SCA, PCI DSS, AML, or sanctions-screening obligations. Those systems produce inputs (SCA result, screening epoch, card-data scope) that the PED MAY require. A clean screening result is not settlement authority for a substituted beneficiary. Operations staff SHOULD log authority_id, instruction_digest, consume time, and settlement_ref together. That tuple is what a scheme or correspondent will ask for after a dispute. Logging the model prompt instead of the digest is not a substitute.

Failure Codes and Denial Behavior Illustrative codes, not IANA assignments: EF-002 NO_FINALITY_AUTHORITY, EF-005 AUTHORITY_ALREADY_USED, EF-006 REPLAY_DETECTED, EF-023 INSTRUCTION_SUBSTITUTION, EF-024 AMOUNT_MISMATCH, EF-025 BENEFICIARY_MISMATCH, EF-026 RAIL_MISMATCH, EF-027 MANDATE_MISS, EF-028 PAYEE_BLOCKED, EF-029 AMOUNT_ENVELOPE, EF-040 SINK_MISMATCH, EF-070 ESCALATION_REQUIRED, EF-080 FAIL_CLOSED. A denial MAY name a remediation (re-quote amount, escalate checker, refresh risk list) but MUST NOT post. Timeout MUST NOT post.

Combined Agent Tool Call and Payment Act When the instruction is born as a model tool call, two objects SHOULD be created from the same canonical arguments: If only the tool dispatcher checks, a handler bug can still post. If only the posting sink checks, a different tool (raw HTTP to the PSP) can still fire. Alternate-path closure requires both for agent-originated value movement.

Security Considerations Threats the sink is intended to make non-completable without current authority include: amount and payee substitution, rail laundering, replay across PSP and core, stolen agent seat, injected invoice, mandate over-use, risk-list staleness, and a second path (manual file drop) that posts the same funds. If the Settlement Sink holds unrestricted posting rights and skips verification, the profile collapses for that path. High-assurance deployments SHOULD keep authority-signing keys off the agent host and SHOULD treat payment class as cold-path default above a ceiling. Idempotency keys supplied by the client are not consume-once authority. An attacker who can change the body and keep the key, or who can mint a new key for the same digest after consume, is out of scope for "we already have idempotency." The consume table MUST be keyed by instruction digest and authority_id, not only by a client string. Sanctions and AML screens are necessary and insufficient. A clean screen of beneficiary-441 does not authorize a post to attacker-992. Screen results SHOULD be bound into PED evidence as an epoch or list identifier so a later list update invalidates unused authority.

Privacy Considerations Instruction objects carry account references and payee identifiers. Implementations SHOULD hash account numbers in logs and SHOULD prefer account_digest in authority objects that leave the posting host. Cross-border acts MUST treat beneficiary jurisdiction as a load-bearing field, not as telemetry.

Implementation Checklist A deployment may call itself aligned with this profile when all of the following are true on the covered rail:

Every post has an authority_id that was consumed atomically with the post.
Instruction digest covers amount, currency, beneficiary, and rail.
Amount, payee, or rail mismatch is a deny, not a log line.
Replay at a second sink fails.
Timeout does not post.
Agent payout tools cannot reach the PSP SDK without the sink function.
Added hot-path latency is measured and sits in the 1-10 ms class or is explained.
Legacy paths that can emit the same clearing message are inventoried; those not yet gated are listed as known alternate paths with a closure date.

IANA Considerations This document requests no IANA actions.

Intellectual Property Note Concepts in this profile are associated with the DAS Protocols family, including International Application PCT/IB2026/055615. IETF disclosure should follow BCP 79 .

Conclusion Rails move money. Agents and APIs now write the instruction. A signature on the channel, a token on the worker, or a mandate on the vendor is not authority for this amount to settle toward this beneficiary on this rail now. Bind the digest. Commit evidence. Consume at the posting sink. A signed instruction is not settlement.